Integrated circuit memories have come into widespread use in many applications, particularly in computer systems. It has been the technological trend to increase the capacity and density of such memories. As manufacturing and design techniques have improved, the cost of memory circuits has decreased dramatically, which has greatly expanded the number of applications and the size of the market. The low cost, large capacity integrated circuit memories now in use are volatile; that is, data stored in the memories is lost when the power is removed. Many applications would be improved if the memories were nonvolatile such that the data would not be lost when power is removed. In certain applications, it is mandatory that the data be retained in the memory. To fill this market, a number of nonvolatile memories have been developed. Among the most common of these now in use is the electronically programmable read only memory (EPROM). However, the nonvolatile memories now available typically have a rather low density of memory storage, are more complex to manufacture than low cost volatile memories, often have a limited lifetime, and are much more expensive than the volatile memories. The need for nonvolatile memory storage in integrated circuits was recognized early, but a low cost solution has not yet been found.
The phenomenon of ferroelectric materials has been known for many years. Such materials have multiple polarization orientation states, which can be selected by the application of an electric field. When a particular orientation state is set in a ferroelectric material, it is retained even when no power is further applied to the material. It is therefore possible to store a particular state in a nonpowered device and then read the state at a later time. Thus, it has been recognized that ferroelectric materials could serve as memory elements in an electrical circuit. An early description of this use is disclosed in U.S. Pat. No. 2,695,396 to Anderson. Since the Anderson patent, there have been more disclosures of circuitry which utilize ferroelectric elements for memory storage. Such patents include 4,144,591 to Brody, 4,149,301 to Cook and 4,360,896 to Brody. However, despite the numerous references to reports of ferroelectric memory devices, it is not known that any device of this type has been introduced into the commercial market. The demand for nonvolatile memories is well recognized and the apparent applicability of ferroelectric materials in the fabrication of such devices has been reported, but numerous problems have been experienced that have blocked the manufacture and production of any practical device. These problems include, among others, the inability to fabricate a reliable ferroelectric material for use in the field of integrated circuits, the use of ferroelectric materials which include contaminants that damage integrated circuitry, unsuitable physical configurations for ferroelectric cells, short lifetime devices, and inordinately high manufacturing costs.
The present invention includes a ferroelectric capacitor, a corresponding memory cell, and a method of manufacture which addresses and overcomes the barriers which have heretofore prevented the production of a reliable, high-density, economical, monolithic ferroelectric memory circuit.